Hitherto, various methods have been proposed for measuring transformation, such as a method making use of radioactive rays, method making use of D.C. magnetic field, and a method which relies upon detection of electromagnetic change caused in the course of transformation as a change in the coil impedance. The measuring method employing the radioactive rays makes use of such a phenomenon that the angle of X-ray diffraction changes due to change from austenite to ferrite. With this method, however, information is available only with the surface area of several tens of microns from the object surface, because of poor permeability of the X-rays in the object, and the measurement is too largely affected by fluttering of the object. The method making use of D.C. magnetic field employs a hole element for measuring the intensity of the D.C. magnetic field applied to the object. This method, however, suffers from a disadvantage in that there is a large non-linearity between the transformation degree and the measured value. In addition, this method permits the measurement of the transformation degree only in the period immediately after the commencement of the transformation because the magnetic field flows only through the surface region of the object when the object has come to exhibit ferromagnetic property.
A description will be made hereinunder as to the transformation degree measuring method relying upon the coil detection of electromagnetic change in the course of the transformation, because the method of the invention also basically relies upon this principle. FIG. 1 shows a system for carrying out this type of measuring method. This system includes an oscillator 1 for producing sine-wave electric current of an appropriate frequency. A detection coil 3 is placed in the close proximity of the upper side of an object 2 to be examined, so as to detect any change in the amount of transformation in the object 2. A reference numeral 4 designates a bridge circuit for detecting any change in the impedance of the detection coil 3, while numeral 5 designates a signal processor for processing the detection signal derived from the bridge circuit 4 into signals of a form easy to use. Numeral 6 denotes a display.
In the operation of this system, the detection coil 3 is placed in the close proximity of a tabular object 2 to be examined, and a continuous sine-wave current is applied to the detection coil from the oscillator 1. Consequently, eddy current is generated in the surface of the object 2, so that the impedance of the coil is changed seemingly.
The amplitude of the eddy current affecting the coil impedance depends on the properties of the object material and other factors, i.e., magnetic permeability, conductivity, distance between the sensor and the object and so forth. When the transformation of the object from austenite to ferrite is proceeding, the magnetic permeability is largely changed because the ratio of ferrite to austenite in the object is increased. Consequently, the amplitude of the eddy current in the surface region of the object is changed to cause a change in the impedance of the detection coil 3 placed in the vicinity of the object. This change in the impedance is detected by the bridge circuit 4 connected to the coil, so that the change in the impedance as an index of the amount of transformation is obtained through the signal processing by the signal processor 5 and displayed on the display 6.
This known system for measuring the transformation degree relying upon a detection coil is sensitive not only to the change in the magnetic permeability of the object but also to other factors, particularly to the distance between the object and the detection coil. It is, therefore, essential to keep a constant distance between the object and the detection coil. In addition, since there is a large non-linearity between the sensor output and the transformation degree it is not possible to measure the transformation degree over a wide range from the beginning to the end of the transformation, although the measurement can be made satisfactorily in a short period immediately after the commencement of transformation.